1. Field of the Invention
The present invention relates to a process for preparing dispersions stabilized with protective colloid using sulfonato-containing polyvinyl acetals as the protective colloid, and to the use of such dispersions, for example, in aqueous binder systems.
2. Description of Related Art
Dispersions are used in many areas of application, for example in the adhesives sector, in the building materials sector, for interior paints and gloss paints, exterior paints and wood glues. With dispersions, a distinction is made between systems stabilized with emulsifiers and those stabilized with protective colloids. Furthermore, a distinction must be made between ionically stabilized and nonionically stabilized dispersions.
For a multitude of processing methods, the dispersions used have to be extremely stable to shearing. Dispersions stabilized with emulsifiers usually do not meet this requirement. This is why dispersions stabilized with protective colloids are used, or at least used additionally.
Dispersions stabilized with protective colloid have been disclosed in "J. Appl. Polym. Sci.", Vol. 40, 333-343 (1990) and in "Polym. Mat. Sci. & Eng.", 51, 172 (1984). A process for emulsion polymerization of ethylenically unsaturated monomers in the presence of hydrophobically modified hydroxyethylcellulose is described in EP-A 0,334,354. However, hydrophobically modified hydroxyethylcelluloses have a tendency to form particle aggregates and to become destabilized, especially in pure acrylate dispersions ("J. Appl. Polym. Sci.", Vol. 40, 333-343, 1990). High grafting rates in the polymerization of acrylate monomers with cellulose ethers lead to insufficient film clarity and dilatancy of the dispersion ("J. Coat. Techn.", Vol. 61 (779), 49, 1989).
A further disadvantage of the known dispersions stabilized with protective colloids is that the desired viscosity for the application has to be adjusted afterwards by addition of thickeners, fillers or else by dilutions.
Often, stability of the dispersions thus adjusted is also problematical. In order to avoid coagulation or settling of the dispersions, specific recipes have to be developed each time depending on the additives used.